1.0 Field of the Invention
The present invention relates to a RF (Radio Frequency) synthesizer and, more particularly, to a frequency locked, digitally tuned RF synthesizer that provides a high speed, broad band, and accurate signal source.
2.0 Description Related to the Prior Art
There are many types of RF (Radio Frequency) synthesizers in present use. These fall into one of four general categories: Phase Locked, YIG tuned, Direct Digital, and Digitally Tuned Oscillator. The phase locked synthesizer usually employs a Voltage Controlled Oscillator (VCO) that is phase locked to some fixed low frequency reference oscillator via either a frequency divider circuit or a narrow band sampling downconverter circuit. The output frequency of the phase locked synthesizer is adjusted by changing the divider circuit characteristics. The phase locked synthesizer design is usually complex, as the frequency divider circuits are required to divide by non-integer ratios, exhibits the frequency stability of the fixed low frequency reference, and, because of the narrow video bandwidths of the phase lock circuit, the phase locked synthesizers tend to be relatively slow in its response (1 ms to 10 ms, typically).
The YIG (Yittrium Iron Garnet) tuned synthesizer provides a wide band (usually up to an octave relative to the input frequency) RF output, and is based on an oscillator using a YIG sphere as a resonator, is oven stabilized, and is tuned by changing the magnetic field applied to the YIG. The accuracy of the YIG tuned synthesizer is poor (typically 0.1% of the output frequency, ±18 MHz at 18 GHz). In addition, the tuning time is relatively slow (100 ms).
The direct digital synthesizer generates an RF output by sequencing through a series of addresses accessed from a RAM (Random Access Memory) that has previously been loaded with a mathematical sine function. The direct digital synthesizer produces a relatively fast RF output, but is limited in frequency resolution and bandwidth. Improvements of the frequency resolution and operating bandwidth typically lead to substantial complexity in the synthesizer design.
The typical digitally tuned oscillator consists of a VCO that is digitally tuned, using a Programmable Read Only Memory (PROM) calibration table to compensate for the nonlinear tuning characteristics of the VCO. To reduce the output frequency error over temperature, the VCO is often oven stabilized. While this produces a fast response (<10 μS), achievement of output frequency accuracy's on the order of 1 MHz requires extensive calibration. Further, the oven stabilization leads to a long-term frequency drift, requiring occasional recalibration. It is desired to provide a RF synthesizer that does not suffer the drawbacks of the prior art. More particularly, it is desired to provide a RF synthesizer having a high speed and a broad band, and serves as an accurate signal source having long term frequency stability.